In density, practitioners use a variety of restorative material in order to create crowns, veneers, direct fillings, inlays, onlays and splint. Composite resins are a type of restorative material which are suspensions of strengthening agents, such as mineral filler particles, in a resin matrix. These materials may be dispersion reinforced, particulate reinforced, hybrid composites or flowable composites. A full discussion of these materials is included in U.S. patent application Ser. No. 09/270,999, entitled "Optimum Particle Sized Hybrid Composite," now pending C. Angeletakis et al. filed on even date herewith (incorporated herein by reference in its entirety).
Highly pure submicron particles are useful in these composite resin materials because they impart the desirable optical properties of high gloss and high translucency. Typically, submicron particles prepared by the commonly employed precipitation or sol gel methods are used to reinforce hybrid composite fillers. However, sol gel preparation methods do not restrict the particle size to at or below the wavelength of visible light and thus do not result in a stable glossy surface in the resin.
While agitator ball mills are known for producing submicron particles, they have previously not been used to produce particles for filler in dental composites because of the impurities which result. The inclusion of impurities in dental composites decreases gloss and translucency. Prior art mills are set forth in U.S. Pat. Nos. 5,335,867; 4,129,261; and 4,117,981, all assigned to Draiswerke GmbH and each incorporated herein by reference in its entirety; and U.S. Pat. No. 5,065,946, assigned to Matsushida Electric Industrial Co. and incorporated herein by reference in its entirety. These prior art mills typically include ceramic or metallic agitators and grinding chambers. During milling, the ceramic or metallic material of the agitator and grinding chamber spalls and becomes co-mingled with the material being ground. In the case of fillers for dental restoratives, these inclusions are unacceptable due to their impact on the optical properties of the restorative. The inclusions may cause decreased gloss due to light scattering and decreased translucency. Draiswerke, Inc., Mahwah, N.J., has applied a polyurethane coating on the agitator and grinding chamber for their PML-H/V machine. The pigment from this coating, however, also contaminates the composites, making them unacceptable for dental use.
The predominant types of milling methods are dry milling and wet milling. In dry milling, air or an inert gas is used to keep particles in suspension. However, fine particles tend to agglomerate in response to van der Waals forces which limits the capabilities of dry milling. Wet milling uses a liquid such as water or alcohol to control reagglomeration of fine particles. Therefore, wet milling is typically used for comminution of submicron-sized particles.
A wet mill typically includes spherical media that apply sufficient force to break particles that are suspended in a liquid medium. Milling devices are categorized by the method used to impart motion to the media. The motion imparted to wet ball mills includes tumbling, vibratory, planetary and agitation. While it is possible to form submicron particles with each of these types of mills, the agitation or agitator ball mill is typically most efficient.
The agitator ball mill, also known as an attrition or stirred mill, has several advantages including high energy efficiency, high solids handling, narrow size distribution of the product output, and the ability to produce homogeneous slurries. The major variables in using an agitator ball mill are agitator speed, suspension flow rate, residence time, slurry viscosity, solid size of the in-feed, milling media size and desired product size. As a general rule, agitator mills typically grind particles to a mean particle size approximately 1/1000 of the size of the milling media in the most efficient operation. In order to obtain mean particle sizes on the order of 0.05 .mu.m to 0.5 .mu.m, milling media having a size of less than 0.45 mm can be used. Milling media having diameters of about 0.2 mm and about 0.6 mm are available from Tosoh Ceramics, Bound Brook, N.J. Thus, to optimize milling, it is desirable to use a milling media approximately 1000 times the size of the desired particle. This minimizes the time required for milling.
Previously, the use of a milling process to achieve such fine particle sizes was difficult due to contamination of the slurry by the milling media. By using yttria stabilized zirconia (YTZ or Y-TZP, where TZP is tetragonal zirconia polycrystal) the contamination by spalling from the milling media and abrasion from the mill is minimized. Y-TZP has a fine grain, high strength and a high fracture toughness. YTZ is the hardest ceramic, and because of this hardness, YTZ will not structurally degenerate during milling. High strength Y-TZP is formed by sintering at temperatures of about 1550.degree. C. to form tetragonal grains having 1-2 .mu.m tetragonal grains mixed with 4-8 .mu.m cubic grains and high strength (1000 MPa), high fracture toughness (8.5 MPa m.sup.1/2) and excellent wear resistance. The use of Y-TZP provides a suitable milling media for providing relatively pure structural fillers having mean particle sizes less than 0.5 .mu.m. Alternatively, glass beads may be used, but because these will abrade, the particular glass used should have optical properties that are the same or similar to the filler material being ground.
Despite some reduction in contamination of the ground filler particulate by the use of YTZ milling media, agitator mills still introduce an unacceptably high level of contamination into dental composites containing the ground filler. Thus, what is needed is an agitator mill that is efficient and produces ground filler particulate that has minimal contamination.